E. T. Turkdogan

Gaseous reduction of iron oxides: Part III. Reduction-oxidation of porous and dense iron oxides and iron

The internal reduction of high-grade granular hematite ore in hydrogen and carbon monoxide, and also the internal oxidation of porous iron granules in CO2-CO mixtures have been investigated. To assist the interpretation of the rate data for porous iron and iron oxides, rate measurements have been made also with dense wustite, previously grown on iron by oxidation. The iron formed by reduction of dense wustite is porous, similar to that observed when porous hematite is reduced. It is found that the rate of dissociation or formation of water vapor or carbon dioxide on the iron surface is about an order of magnitude greater than that on the surface of wustite. The results of the previous investigations using dense iron and wustite are in general accord with the present findings. The rate of reduction of hematite increases with increasing pore surface area of the reduced oxide. The results indicate that the rate of reduction of granules is controlled primarily by the formation of H2O or CO2 on the pore walls of wustite. The specific rate constants evaluated from internal reduction, using the total pore surface area, are about 1/50 to 1/100 of those for dense wustite. These findings indicate that with porous wustite or iron, the effective pore surface area utilized is about 1 to 2 pct of the total pore surface area. The rate of reduction in H2-CO mixtures is in accord with that derived from the rate constants for reduction in H2 and CO.

Gaseous reduction of iron oxides: Part I. Reduction of hematite in hydrogen

The reduction of high-grade hematite ore in hydrogen has been investigated. There is an unusual temperature effect for small granules with a dip in the rate at about 700°C, similar to those reported by previous investigators for different types of iron oxides. The particlesize effect on the time of reduction suggests that there are three major limiting rate-controlling processes: i) uniform internal reduction, ii) limiting mixed control and iii) gas diffusion in porous iron layer. Processes (ii) and (iii) are special cases of a so-called topochemical mode of reduction associated with the formation of product layers. Unidirectional reduction experiments revealed the significant role played by gas diffusion in porous iron layer as a rate-controlling process. The effective H2-H2O diffusivity in porous iron derived from, the reduction data is found to decrease markedly with decreasing reduction temperature. This is consistent with the fracture surfaces of porous iron as viewed by scanning electron microscopy. The present interpretation of the rate of reduction of hematite ore is found to apply equally well to previously published data on the hydrogen-reduction of natural and synthetic hematite pellets.

EFFECT OF CARBON MONOXIDE ON THE RATE OF OXIDATION OF CHARCOAL, GRAPHITE, AND COKE IN CARBON DIOXIDE.

Abstract The rate of oxidation of coconut charcoal, electrode graphite and metallurgical coke granules in CO2-CO and CO2-CO-He gas mixtures at various pressures was measured within the range of 700°–1400°C. The rate measurements were made under conditions such that almost complete internal burning prevailed, at temperatures below 1000° or 1200°C depending on the type of carbon. Carbon monoxide is found to have a pronounced retarding effect on the rate of oxidation in carbon dioxide. This retarding effect is attributed to the strong chemisorption of carbon monoxide on the pore surface of carbon. The experimental results (for the case of almost complete pore diffusion) can be interpreted reasonably well in terms of a reaction mechanism involving two consecutive rate-controlling reactions in series: 1. (1) dissociation of CO2 on the surface of carbon and 2. (2) formation of CO on the surface of carbon. The reactions in series are analogous to resistances in series; the rate is controlled by the reaction step which exerts most of the resistance to the overall reaction. At low CO contents, resistance of (2) ⪢ resistance of (1) and at CO contents above about 10 per cent, resistance of (1) ⪢ resistance of (2). The apparent heat of activation for rate (1) is 60 kcal/g-atom C and for rate (2) is 69 kcal/g-atom C. Carbon monoxide has a two-fold poisoning effect: 1. (a) covering of the surface site due to strong adsorption and 2. (b) increasing the activity coefficient of the activated complex for the dissociation of CO2; hence CO changes the rate-controlling mechanism from (2) to (1). The temperature dependence of the rate parameters is obtained from the rate data for these two mechanisms. In addition, adsorption isotherms are derived indirectly from the rate data for adsorption of CO on the surface of carbons.

Kinetics of oxidation of graphite and charcoal in carbon dioxide

Abstract The rates of oxidation of electrode graphite and coconut charcoal granules in carbon dioxide (with or without dilution with inert gases) were measured over a pressure range of 10−3 to 40 atm at temperatures from 700 to 1300°C. The effects of temperature, pressure, particle size and type of carbon on the extent of internal burning are demonstrated. To insure almost complete gas diffusion in the pores of the carbon, it is shown that with increasing temperature and pressure the particle size must be decreased. Under these conditions, the rate is found to be proportional to the square root of the pressure of carbon dioxide and it is concluded that the rate is controlled primarily by the formation of carbon monoxide from chemisorbed oxygen and carbon on the pore surface. This reaction mechanism for carbon dioxide, for which the surface coverage with oxygen atoms is believed to be small, is shown to apply to the oxidation of graphite in pure oxygen for which almost complete surface coverage with oxygen may be assumed.

PORE CHARACTERISTICS OF CARBONS.

Abstract The pore characteristics of carbons, e.g. graphites, coke, and charcoals, have been investigated by utilizing X-ray and microscopic techniques and by measuring pore volume, pore surface area, and the corresponding effective diffusivities of CO–CO 2 and H 2 –H 2 O. The large surface area of carbons is shown to be due to the presence of micropores which become extensive in charcoals. A method is developed for the determination of surface areas from the BET technique using nitrogen (− 195°C) and carbon dioxide (− 78°C) adsorption isotherms at low relative pressures. Several techniques are used in determining the effective diffusivities of CO–CO 2 and H 2 –H 2 O. It is shown that over a wide temperature range (18–900°C) and at 1 atm pressure and above, the diffusive flux is that due primarily to normal (molecular) diffusion. The pore volume, pore surface area, and effective diffusivity increase with increasing weight loss during internal oxidation. These changes are not reflected in the measurements of the rate of oxidation. The results of our present and previous investigations indicate that for a given type of carbon the effective surface area (due primarily to large pores of 1 μm dia. or larger) does not change much with oxidation up to at least 20 per cent weight loss. The difference in the rate of oxidation between types of carbon is attributed essentially to the difference in the effective surface areas.

Gaseous reduction of iron oxides: Part II. Pore characteristics of iron reduced from hematite in hydrogen

The pore structure of iron reduced from hematite ore by hydrogen is charactertized from measurements of pore volume, pore area and effective gas diffusivity. The measured connected pore volume within the range 0.22 to 0.34 cu cm per g is about the same as the total pore volume; that is, most of the pores in the reduced iron are interconnected. The pore area measured by the BET technique increases with decreasing reduction temperature,e.g. from 0.1 sq m per g at 1200°C to 39 sq m per g at 200°C. The effective H2−H2O diffusivity, measured directly at 600°C after reduction of the ore in hydrogen at the same temperature, is in excellent agreement with that derived from the reduction data. The pore-diffusivity measurements were also made at room temperature using iron samples reduced in hydrogen at 800° and 1000°C. On the basis of the pore properties measured, it appears that the reduced iron has a regular pore structure which becomes finer with decreasing reduction temperature. The effective diffusivities computed on the basis of a simple pore structure are found to be in accord with those derived previously from the reduction data.

RATE OF OXIDATION OF GRAPHITE IN CARBON DIOXIDE.

Abstract Experimental data are presented on the rate of oxidation of electrode graphite spheres ( 1 8 in. to 7 8 in. dia.), blocks and sized granules (8 to 30 mesh) in carbon dioxide-argon mixtures at one atmosphere total pressure and in carbon dioxide at pressures up to 40 atm. The effects of size, pressure and temperature on the extent of internal burning are demonstrated. It is found that internal burning occurs throughout and that the rate is controlled by the chemical reaction of gas and carbon at one atm CO 2 for spheres less than 0.2, 0.3 or 0.6 cm dia. for 1100, 1000 and 900° respectively. For larger spheres or at higher pressure or temperature, internal burning occurs only in a zone near the surface—that is, pore diffusion is incomplete.

CATALYTIC OXIDATION OF CARBON.

Abstract The rate of oxidation of iron-impregnated electrode graphite granules ( 2 -CO mixtures was measured at temperatures 700 to 1000°C and at pressures 0.03 to 1.0 atm. Some measurements were made also with iron-impregnated coconut charcoal and coke. In air oxidation experiments, graphite granules impregnated with silver, copper, chromium, zinc, nickel, cobalt and iron were used. The rate of oxidation in CO 2 -CO mixtures increases by several orders of magnitude when graphite is impregnated with iron, e.g. with 2.1% Fe-graphite the rate in CO 2 is a factor of about one million greater than that for iron-free graphite. At 800°C, the rate increases almost linearly with the iron content. After 20 to 50% oxidation the catalytic effect of iron diminishes even in gas mixtures where iron cannot be oxidized. However, upon H 2 -treatment the catalytic effect can be revived. The dependence of the rate on gas composition suggests that there might be strong CO 2 and CO adsorption on the pore walls of graphite impregnated with iron and that for a given total pressure the rate of oxidation in CO 2 -CO mixtures is proportional to the partial pressure of CO 2 . That is, the rate of oxidation is controlled by the dissociation of CO 2 with an apparent heat of activation of 87.6 kcal.

Gaseous reduction of iron oxides: Part IV. mathematical analysis of partial internal reduction-diffusion control

In this mathematical analysis of gaseous reduction of iron oxides, the partial internal reduction of the porous oxide and gas diffusion in the porous iron layer are considered simultaneously in deriving the rate equation. The rate equation, derived by partly analytical and partly numerical solutions, is well substantiated by the experimental results obtained previously. The following parameters, determined previously, are used in the application of the rate equation: i) specific rate constant for the gas reaction on the pore walls of wustite, ii) pore surface area of wustite, iii) effective gas diffusivity in the porous wustite formed during reduction of hematite, and iv) effective gas diffusivity in the porous iron layer. The effective depth of the internal reduction zone at the wustite-iron diffuse interface increases steeply with the progress of reduction beyond about 50 pct O removal. For reduction of 1 to 2 cm diam hematite spheroids in 100 pct H2, the gas composition at the diffuse iron-wustite interface is within 10 to 20 pct of that for the iron-wustite equilibrium; beyond about 50 pct O removal, the rate of reduction is controlled primarily by gas diffusion in the porous iron layer. From the mathematical analysis it is found that the relative depth of internal reduction increases with decreasing particle size and increasing temperature.

Incomplete pore diffusion effect on internal burning of carbon

Abstract The internal burning of carbon, e.g. graphite or coke, controlled simultaneously by the counterdiffusion of CO 2 and CO in the pores and the chemical reaction on the pore walls, is investigated through mathematical analysis. Because of the strong poisoning effect of CO on the rate of oxidation of carbon, incomplete pore diffusion markedly reduces the depth of internal burning. The effects of particle size, temperature and pressure on the rate of oxidation of graphite and coke in CO 2 -CO mixtures derived from the mathematical analysis are found to be in reasonable accord with present and previous experimental observations. At the limiting case of incomplete internal burning, i.e. pore diffusion control, the rate per particle is found (1) to be proportional to the external geometrical surface area, (2) to be independent of pressure, and (3) to have an apparent heat of activation half of that for complete internal burning (chemical reaction control).